Paper No. 5
Presentation Time: 8:00 AM-12:00 PM
3.2 GA EOLIAN SAND SHEET DEPOSITS FROM THE MOODIES GROUP, BARBERTON GREENSTONE BELT, SOUTH AFRICA
The oldest reported evidence of wind erosion is recorded in ventifacts associated with placer deposits in the 2.9 Ga Witwatersrand Supergroup, South Africa. The oldest reported evidence eolian deposition is sand sheet deposits composed of eolian stratification from the 2.6 Ga Minas Supergroup of Brazil. This paper describes the oldest recognized evidence of eolian processes in the form of sand sheet deposits from the 3.2 Ga Moodies Group in Barberton Greenstone Belt, South Africa, and indicates the development of significant wind regimes during the Archean. The sand sheet deposits are developed in the lower Moodies Group in the central part of the belt and are composed of stacked inversely graded strata. The base of an individual stratum consists of fine- to medium-grained quartz sandstone that passes vertically into very coarse- to coarse-grained quartz sandstone. Pressure solution cleavage is locally developed and is restricted to the basal fine-grained sandstones. This cleavage is parallel to the stratification and does not hinder grain size measurement. Laterally the inclination of the strata commonly increases. Strata compose sets up to 3 to 5 m thick. The inversely graded stratification is interpreted as the product of wind-ripple migration. The coarse grain size in conjunction with the low-angle of inclination of the strata are best interpreted as the product of sand sheet development. Within this facies, a channel composed of steeper (> 8 degrees) normally graded stratification is present and interpreted as an incursion of fluvial channel into the sand sheet. Eolian set boundaries may reflect changes in sand supply, wind regime, ground water level, eolian accumulation rate or any combination of factors. These factors were likely controlled by climate fluctuations.